Light emitting diodes (LEDs) are widely accepted as light sources in many applications that require low power consumption, small size, and high reliability. Diodes that emit light in the yellow-green to red regions of the visible spectrum contain active layers formed of an AlGaInP alloy, often grown on a GaAs substrate. Since GaAs is absorbing, it is typically removed and replaced by a transparent substrate.
US 2009/0108276 states “[t]here are a number of known difficulties with currently used yellow-red AlInGaP-based light emitting devices. For example, they suffer from low internal quantum efficiency and poor temperature stability in the yellow-red range, which is usually attributed to poor electron confinement. In addition, the conventional procedure for removing the light absorbing GaAs substrate and wafer bonding a transparent substrate or reflective layer to the formed layer has a low yield and adds several relatively expensive processing steps, thus resulting in high costs.”
FIG. 1 illustrates an LED structure described in paragraph 21 of US 2009/0108276. In FIG. 1, “the LED structure may comprise a GaP substrate [10] over which is formed a GaP buffer layer [12], over which is formed an active region [14] comprising interleaved layers of a GaP barrier layer and a InmGa1-mNcP1-c layer where 0.001<c<0.05 and 0≦m≦0.4 active layer, over which is formed a GaP cap/contact layer [16]. In some embodiments of this specific structure, the GaP substrate [10] and GaP buffer layer [12] may be n-type and the cap/contact layer [16] may be p-type.”